1. Field of the Invention
This invention relates to pressure-sensitive adhesive compositions. More particularly, the invention relates to a pressure-sensitive adhesive composition comprising a low molecular weight polyisoprene modified by maleic anhydride or a derivative of maleic anhydride, a cross-linking agent therefor and a tackifier.
2. Description of the Prior Art
The conventional method of manufacturing pressure-sensitive adhesive compositions for various final products such as pressure-sensitive adhesive tapes comprises dissolving a rubber-type high molecular weight polymer, a tackifier and other components in an organic solvent such as toluene, hexane or a hydrocarbon rubber-solvent to prepare a solution with a total solids content of a few percent to tens of percent, coating a backing member with the solution and drying the same to remove the organic solvent therefrom. In such a manufacturing process, the large quantity of solvent has to be removed by the drying operation which, of course, entails a significant economic loss due to the lost solvent and the thermal energy consumed. Meanwhile, government regulations on working environments and against atmospheric pollution have become increasingly rigorous and will certainly be made more stringent in the future. In view of those circumstances, it is the ultimate goal of the adhesive industry to ensure consistently high solids concentrations of the system throughout the process of manufacture of pressure-sensitive adhesive compositions or, for that matter, to provide a completely solventless system of manufacture. Such a desirable manufacturing technology cannot be established from improvements or innovations in equipment alone but requires studies and solutions from the raw material side as well. There is already disclosed in U.S. Pat. No. 3,160,595 a pressure-sensitive adhesive composition consisting of depolymerized polyisoprene rubber, p-quinonedioxime and lead dioxide but the composition is not only inadequate in cohesive force but, because of its being a depolymerized product, has of necessity been thermally discolored. A few other compositions have also been recently developed for the stated purpose but their performances are not completely satisfactory. Thus, although various attempts have so far been made to manufacture pressure-sensitive adhesive compositions using a low molecular weight liquid rubber or a derivative thereof, for instance, such compositions have proved still unsatisfactory, apparently due to the fact that the liquid rubber is based on butadiene, that its molecular weight is too low, i.e., a few thousand at the maximum, and that the distribution of functional groups within the molecular chain is not appropriate.